Attempts have been made to develop effective proportional solenoid valves suitable for use in flow modulation. In such applications, a key problem is nonlinearity of the relationship between applied force and displacement for the armature and attendant mass which is moved in order to provide flow modulation. This has led to designs such as those exemplified by U.S. Pat. No. 4,690,371 Bosley et al in which pressure sensing arrangements and feedback circuits are employed to compensate for thenonlinearity.
An approach different from the forementioned designs is disclosed in U.S. Pat. No. 4,988,074 Najmolhoda. In the '074 patent, an armature 22 is suspended by two plate springs 24, 26, and a coil spring 42 is used to bias the armature in a direction which renders the valve normally closed. The force exerted by the coil spring 42 must be sufficient to overcome both the magnetomotive force exerted on the armature by a permanent magnet 34 and the force exerted on the same by the pressurized fluid with which the valve operates. In many applications, this approach is unsatisfactory for reasons which will become evident from the following description.
An objective of the invention is to provide a proportional solenoid valve exhibiting highly linear response over the range of movement associated with its armature, spool, or other flow modulating element.
A further objective of the invention is to provide such a valve with relatively small radial dimensions.
A still further objective is to provide a solenoid valve that operates in a manner which is substantially independent on the pressure of the fluid which flows through the valve.